Regarding disproportionation of polysiloxane, a method in which an acidic or alkaline catalyst is used is generally known. However, such an acidic or alkaline catalyst-aided method is not advantageous for industrially stable production because of the severe reaction conditions. Formation of cyclosiloxane by heat decomposition of polysiloxane is known as an example of disproportionation method under neutral condition, but this method requires an extremely high temperature of about 300.degree. C. so that this cannot also be regarded as an advantageous method for industrially stable production. As another disproportionation method under neutral condition, a method in which a transition metal catalyst is used is known, but since this method requires the use of a catalyst of Pt, Pd or the like expensive metal, this cannot be said as an industrially advantageous method when such a costly point of view is taken into consideration.
Cyclic oligosiloxane compounds are used as materials for the production of high molecular weight polysiloxane compounds in the field of silicone industry. Also, a cyclic oligosiloxane containing Si--H group is used as a material for the production of room temperature crosslinking type silicone rubber which is used for example in sealant in the field of silicone industry. The room temperature crosslinking type silicone rubber is obtained by crosslinking the cyclic oligosiloxane containing Si--H group with a polysiloxane containing vinyl group through their reaction in the presence of a platinum catalyst. The cyclic oligosiloxane containing Si--H group is also used as a material for the production of an organic functional group-linked oligosiloxane which is used for example as an adhesive property increasing agent. The organic functional group-linked oligosiloxane is obtained by allowing the cyclic oligosiloxane containing Si--H group to react with a vinyl group-containing organic group in the presence of a platinum catalyst.
As a generally known method for the production of cyclic oligosiloxane, a method of hydrolysis condensing organosilanes having two hydrolyzable groups on its silicon atoms such as dimethyldichlorosilane or the like compound. However, this method is not advantageous for its industrially stable production, because the reaction system becomes acidic severe condition under such hydrolysis condensation condition.
Particularly, in the case of the production of the cyclic oligosiloxane containing Si--H group, the Si--H group has considerably high reactivity under such acidic condition and therefore reacts with water or silanol group coexisting in the reaction system, so that yield of the compound of interest becomes low. Accordingly, the following methods (a) and (b) have been proposed for dissolving this problem.
Namely, (a) a method in which methyldichlorosolane is hydrolyzed in the presence of a mixed solvent of tetrahydrofuran with a hydrocarbon solvent (JP-A-6-80680; the term "JP-A" as used herein means an "unexamined published Japanese patent application") and (b) a method in which dichlorodisiloxane is hydrolyzed in the presence of t-butyl alcohol (JP-A-7-285974).
However, it is necessary to use benzene as the hydrocarbon solvent in the aforementioned method (a) in order to obtain a cyclic oligosiloxane as the product with a high yield, but industrial realization of this method may cause a problem in terms of the safety for benzene. Also, dichlorodisiloxane to be used as a material in the aforementioned method (b) is not a material commercialized as a generalized product, so that availability of this material generally becomes a problem when this method is carried out, as well as a problem of lacking in the flexibility of industrial techniques.
In addition, chlorosilane is hydrolyzed in each of the methods (a) and (b), but it is known that hydrogen chloride is formed as a by-product by this reaction and the product is contaminated by a small amount of hydrogen chloride. However, when such a product containing hydrogen chloride is used for example in electronic materials, the final product containing ion components such as chloride ion is not desirable from the viewpoint, for example, of the corrosion of electrodes. Thus, the products manufactured by such methods are not suited for their application to electronic materials and the like, and their industrial availability is reduced. Also, the reaction is carried out under strongly acidic condition in each of the methods (a) and (b), so that not only the steps become complex, such as adjustment of pH in the after step, but also it causes a problem in that caution is required in handling the material.
As other methods for the production of cyclic oligosiloxane, certain methods have been proposed in which a linear polysiloxane or a high polymerization degree cyclic polysiloxane is subjected to its reaction under various conditions in the presence of an acidic or alkaline catalyst.
Examples of the method which uses an acidic catalyst include (c) a method in which polysiloxane containing Si--H group is subjected to the reaction in the presence of water and activated clay (JP-B-54-13480; the term "JP-B" as used herein means an "examined Japanese patent publication"), (d) a method in which methylhydrogenpolysiloxane is subjected to the reaction by heating it in the presence of an acid catalyst (JP-B-55-11697), (e) a method in which organopolysiloxane is subjected to the reaction by allowing it to contact with a heated fixed catalyst bed under a reduced pressure (JP-A-2-129192), (f) a method in which the reaction of methylhydrogenpolysiloxane is carried out in the presence of a high boiling point organodisiloxane (JP-A-7-242678) and (g) a method in which organohydrogenpolysiloxane is subjected to the reaction in the presence of aluminum chloride (JP-A-7-316167).
However, the aforementioned method (c) has an industrially serious problem of causing gelation of the reaction system due to high reactivity of the Si--H group with water under acidic condition. The aforementioned methods (d) and (e) require a considerably high temperature of from 250 to 500.degree. C. for the reaction, thus causing a problem when used in an industrial scale. In the aforementioned method (f), it is necessary to use a high boiling point disiloxane as a side material in addition to the main material. However, such a special disiloxane is not a material commercialized as a generalized product, so that this method lacks in the flexibility of industrial techniques in terns of the availability of the material when this method is carried out. It also is not industrially advantageous from the viewpoint of cost.
In addition, each of the aforementioned methods (c) to (g) uses an acid catalyst. Particularly, the aforementioned method (g) uses aluminum chloride as the catalyst having considerably strong acidity. Also, since Si--H group is basically unstable against water and the like under acidic condition, it is not desirable to use an acid catalyst in the Si--H group-containing system of these methods; for example, when the reaction system is contaminated by even a small amount of water for example from the material or air, the Si--H group reacts with water to decrease yield of the cyclic oligosiloxane as the product and also to cause gelation of the reaction system. In consequence, the aforementioned methods (c) to (g) which use acid catalysts are techniques that have practical problems from the viewpoint of stable industrial production.
Examples of the method in which an alkaline catalyst is used include (h) a method in which carbonate of an alkali metal is used as the catalyst (JP-B-45-15036) and (i) a method in which an alkali metal silanolate is used as the catalyst (JP-B-33-2149).
However, since these methods use alkaline catalysts, not only the steps become complex, such as adjustment of pH in the after step, but also caution is required in handling the materials, similar to the case of acid catalysts, so that they have a problem in terms of stable industrial production. In addition, since the Si--H group in the Si--H group-containing system is considerably unstable under alkaline condition, these methods cannot substantially be used. Thus, the aforementioned methods (h) and (i) which use alkaline catalysts are techniques that have practical problems from the viewpoint of stable industrial production and also lack in flexibility.
In consequence, in order to resolve the aforementioned problems, the present invention aims at providing a method for the production of cyclic oligosiloxane, which is practical in carrying out industrial production because a material available with low cost is used, the reaction can be carried out under moderate conditions, namely neutral condition and relatively low temperature, and, particularly, gelation of the reaction system does not occur in the case of a system which contains Si--H group.